Fibroblast Growth Factor 18 (FGF-18) is a member of the Fibroblast Growth Factor (FGF) family of proteins, closely related to FGF-8 and FGF-17. Members of the FGF family are characterized by heparin-binding domains. Such a putative heparin-binding domain has been identified for FGF-18. It is postulated that receptor-mediated signaling is initiated upon binding of FGF ligands complexed with cell-surface heparin sulfate proteoglycans.
It has been shown that FGF-18 is a proliferative agent for chondrocytes and osteoblasts (Ellsworth et al., 2002; Shimoaka et al., 2002). FGF-18 has been proposed for the treatment of cartilage disorders such as osteoarthritis (OA) and cartilage injury (CI), either alone (WO2008/023063) or in combination with hyaluronic acid (WO2004/032849).
Pharmaceutical compositions comprising an FGF polypeptide are known from the art. WO2012/172072 describes a freeze-dried formulation containing FGF-18, wherein said composition comprises FGF-18, a buffer, a poloxamer surfactant and a sugar as stabilizing agent. Said FGF-18 freeze-dried formulation is showing promising results in the treatment of OA or CI. The current dosing regimen, using said freeze-dried formulation, is a treatment cycle of once weekly injection for 3 weeks. The treatment cycle can be repeated.
In the case of CI, the main drawback of the current formulation is that, once injected intraarticularly (i.a.), the presence of FGF-18 in the synovial fluid may also induce uncontrolled cartilage growth in healthy areas. This can, of course, induce unwanted effects such as reduced joint mobility. The delivery of FGF-18 selectively at the level of the target site could promote the cartilage growth only in the damaged area. In particular, the delivery of FGF-18 at the level of the damaged area could be highly beneficial for the treatment of CI coupled with microfracture techniques. Microfracture is an articular cartilage repair surgical technique that works by creating small fractures in the underlying bone. This causes the release of pluripotent mesenchymal stem cells from the bone marrow (Ringe, J. et al., 2012). Filling the cartilage hole with an injectable gel containing FGF-18 would direct cells within the gel that would then act as mechanical supports for cell growth and drug reservoirs at the same time. For this reason, it would be preferable if FGF-18 is not released from the gel but it stays entrapped in the matrix.
A typical approach in tissue engineering is the confinement of growth factors in a 3D matrix, i.e., a scaffold, that can be either implanted or injected, depending on the mechanical properties, in order to assume the shape of the acceptor site. Mandatory characteristics of the scaffold are biocompatibility and resorbability. Additionally, scaffolds must be able to provide cells the ideal environment to grow, proliferate and reform the damaged tissue. Ideally, the matrix should resemble the same mechanical properties as the original tissue and should present a microporosity able to host cells (interconnected pores with a sufficient size) (Tessmar and Göpferich, 2007).
Hydrogels are three-dimensional networks of hydrophilic polymer chains able to absorb and retain large amounts of water. Their main feature is that they are able to swell or shrink but not dissolve in aqueous media. Therefore, it is possible to entrap in their matrix an active molecule (Active Pharmaceutical Ingredient, i.e., API) that is then slowly released or retained, depending on the presence of specific interactions between the matrix and the API (Lo Presti et al., 2011). The advantage of the use of injectable hydrogels for treating a cartilage disorder is the possibility to inject the scaffold by arthroscopy in the cartilage defect, without the need of any invasive surgery making use of solid scaffolds.
Among the diverse hydrogels that are already known, some formulations are based on polymers able to undergo the gelling process in response to a particular physical or chemical stimulus. These are present as viscous injectable liquids that, once injected, turn to macroscopic gels in response to environmental stimuli at the site of injection, such as changes in temperature, pH or ionic strength. The composition of the formulation can be tuned in order to obtain hydrogels with different characteristics, such as viscoelastic properties, microporosity, etc. (WO2008/063418; Lo Presti et al., 2001; C. Dispenza et al., 2011).
Hydrogels of natural polymers, particularly polysaccharides, have been widely used for their unique advantages, such as nontoxicity, biocompatibility, biodegradability, and abundance. Natural polymers including collagen, gelatin, glycosaminoglycans, and derivatives thereof often possess a high affinity for proteins. A large number of biopolymers possess the property to self-structure upon temperature or ionic variation.
Xyloglucans are a major class of structural polysaccharides found in the primary cell walls of higher plants. When xyloglucan is partially degalactosylated (Deg-XG), it becomes temperature-responsive (thermosensitive): it can form physical, reversible gels with temperature variations in aqueous solutions. Degalactosylation of xyloglucan is achieved with β-galactosidase (Rilton et al., 2011). Degalatosylated xyloglucans present some advantages over other currently available in-situ gelling systems: the gelation does not require the presence of divalent cations and it is not affected by the charged nature of the drug; the gel forms in few minutes, depending on the concentration of polymer in solution and temperature (Shirakawa et al., 1998).
When preparing a pharmaceutical composition comprising a bioactive protein, said composition must be formulated in such a way that the activity of the protein is maintained for an appropriate period of time. A loss in activity/stability of the protein may result from chemical or physical instabilities of the protein, notably due to denaturation, aggregation or oxidation. The resulting products may thus be pharmaceutically unacceptable. Although the use of excipient(s) and/or hydrogels is known to increase the stability of a given protein, the stabilizing effects of these excipients is highly dependent on the polymer in the gels, the nature of the excipients and the bioactive protein itself.
There remains a need for further formulations containing FGF-18 as an active ingredient, wherein said formulations, while keeping the bioactivity of the active ingredient and being suitable for use in injection, preferably for intra-articular injection, allow reduction of the number of injections needed for the treatment. Such a characteristic would allow the reduction of the risk of infections and would increase the patient's convenience. Said formulations could be useful for administration to a patient for the treatment of a cartilage disorder, such as osteoarthritis or cartilage injury.